There were 2 different designs each with 2 variants. They were all rebuilt from coaches built to diagram T38. Diagram T38 was a 4 compartment brake with the guards compartment in the centre of the coach. Conversion to T25 involved converting 2 compartments to a luggage area. The former compartment door of the end compartment was converted to a pair of double doors. The door to the other compartment and the windows were replaced with wooden panels so that the coach looked as if it had been originally built that way.
Conversion to T27 was very similar except that all of the doors and windows in the converted compartments were boarded up.
Diagrams T26 and T28 were T25 and T27 repectively with the guards lookout removed. These are my drawings of these designs based upon information in the Lot Book at York.